Read Ebook: Pottery decoration under the glaze by McLaughlin M Louise Mary Louise

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ays, but ordinarily may be said to be about one-eighth. A piece of ware, therefore, made from clay which exhibits this degree of shrinkage, would be one-eighth smaller after firing than before. The shrinkage is also modified by the degree of heat to which the clay has been subjected in firing, and there will sometimes be a perceptible difference in the size of two pieces of ware, made from the same clay, and of equal size before firing, after having been fired at different temperatures.

The length of time consumed in firing, varies with the qualities of the different wares. In the manufacture of white earthenware, the firing lasts from thirty to forty hours, while the more common kinds of ware require less time. In kilns of ordinary size, a thousand dozen pieces of ware are frequently fired at once. Of course, considerable time is required to place the seggars containing the ware in the kiln. Some hours must elapse before the kiln and its contents will become cool enough to permit its being opened with safety, so that the firing of hard-baked pottery may require two or three days for its completion.

The first firing completed, the ware is said to be in the "biscuit," and is then ready to be glazed. This matter of glazing is a very important one, and the success of the whole very largely depends upon the manner in which the operation is performed. To insure a successful result there must be the nicest adaptation of the materials composing the glaze to the body of the ware. The glaze for each kind of ware must be suited to its especial characteristics, and it can therefore be imagined that the number of glazes in use is very large. Each pottery has its own glaze and the variety is infinite.

The glaze used upon the finer kinds of earthenware consists of materials similar to those of which the body of the ware is made, with the addition of boracic acid , which with a little lead renders the glaze fusible. In the lower grades of ware a larger amount of lead is used for a flux, and this causes the glaze to fuse at the comparatively low temperature at which these wares are fired. A glaze made of materials that fuse at an unusually low temperature is called a "soft" glaze. Some glazes are so soft that, when fired, they can be easily scratched by the point of a steel instrument. The glaze of good and durable wares is, however, so hard, that the point of the sharpest knife will make no impression on its surface.

The materials of which the glaze is made, are combined in the proper proportions and diluted to form a liquid of about the consistency and the appearance of cream. The piece of ware is then dipped carefully into the liquid, and so skillfully manipulated that it is completely covered with a coating of the glaze of the necessary thickness.

After having been covered with the glaze, which, before firing, has the appearance of an opaque white paint, the ware is ready for the second firing. This is done in what is called the "gloss" kiln, in which the heat is not brought to such a high degree as in the biscuit kiln, but is sufficient to fuse the glaze, and cause it to form a glassy, transparent surface, which should completely cover the body of the ware, and present an equally brilliant appearance in every part.

If the glaze is not suited to the body of the ware, and does not shrink equally with it, its surface will soon present a network of fine cracks. A glaze in this condition is said to be "crazed." This, among potters, is considered a serious fault, and in ware intended for cooking or table use, is certainly very undesirable.

Old Japanese crackle-ware, in which this condition is a distinguishing feature, is, however, much esteemed, and many, indeed, the majority of the Japanese earthernwares, both ancient and modern, display a surface of fine-crackled glaze, which may not be said to detract from their value as articles of ornament.

A similar effect is also seen in old pieces of glazed ware which have seen long service as cooking utensils, in which case it has been the natural result of the usage to which they have been subjected.

In some modern wares, both French and English, decorated under the glaze, this defect is seen. In these cases it has been caused by the use of a softer glaze than the body of the ware demanded. This expedient has been resorted to in order that the brilliancy and beauty of the colors might be preserved. These articles being intended only for ornament, beauty is more of an object than durability, and this defect of glaze may be permissible as the means of obtaining more brilliant effects. In certain methods of decoration this may be a necessity, but these cases are exceptional and experience leads me to believe that it is possible to obtain beauty of coloring with a glaze which will remain intact. It is to be hoped, however, that with the improvements in making colors the time may come when it will be possible to obtain colors which will retain their beauty under the degree of heat necessary to the production of an article which a practical potter would call a perfect piece of ware.

A certain delicacy of tint and firmness of outline characterizes overglaze painting, which is easily recognized by experts, but those who can not distinguish between the two methods by this means may ascertain to which class the decoration belongs by looking aslant the surface. The glaze upon the painted portions of a piece of ware decorated over the glaze, will not, even after the most perfect firing, equal the glaze upon the uncovered portions in brilliancy, and there will probably be certain inequalities of surface between the painted and unpainted parts which will be revealed to the touch.

If the decoration has been executed under the glaze the surface will be uniformly covered with a brilliant glaze. Of the two, underglaze painting is probably the most difficult, as the colors are more liable to change under the action of the great heat to which the ware must be subjected, and the final results being, therefore, somewhat uncertain, can not be accurately counted upon, until experience has been gained from repeated failures. On the other hand, overglaze painting, while not subject to the changes produced by the fire, to so great an extent, is more difficult as regards the manipulation of the painting upon the glazed surface. The facilities for the practice of overglaze painting are greater than those afforded for painting under the glaze, which last, requires not only the handling of an artist who has acquired facile use of the brush and some experience of pottery clays and colors, but also the assistance of an intelligent and skillful potter.

Let us not, however, laud one method of painting at the expense of the other. Both are good in their way, and confined within their proper limits, have beauties peculiarly their own. Overglaze painting possesses a delicacy of effect and a variety of color which the underglaze decoration can not rival. The latter is, however, the most artistic as well as the most effective, and lends itself more readily to the uses of decorative art and to the modern taste in color.

It is this latter method of decorating pottery which we will now consider in some of its various forms.

COLORS FOR PAINTING UNDER THE GLAZE.

Colors for painting under the glaze are specially prepared for the purpose from various metallic oxyds. As mentioned in the previous chapter, the colors used under the glaze have less flux or fusible matter in their composition than those used for painting overglaze: this is necessary, because in the former the colors are expected to bear a much greater degree of heat in firing than in the latter. They are so prepared as not to fuse except at the temperature required in the manufacture of the pottery to which they are applied. The variety of underglaze colors is less than that to be found in overglaze colors, as the oxyds from which colors may be made which will stand so great a degree of heat, are few in number.

The oxyds from which the colors are prepared are called the bases of these colors. The colors used in underglaze painting are made from the following bases:

Blue from the oxyd of cobalt. Co. O.

Green from the sesquioxyd of chromium. Cr^2. O^3.

Browns from the sesquioxyd commonly called the peroxyd of iron, Fe^2. O^3., and from the sesquioxyd of manganese. Mn^2. O^3.

Yellow from the compound of titanium with oxygen, called titanic acid. Ti. O^2.

Black from the oxyd of uranium.

Red from the suboxyd, or red oxyd of copper. Cu^2. O.

We may add, upon the authority of M. Debette, that "pure black is obtained by taking 1 part of oxyd of uranium diluted in 22 parts of glaze; ordinary black with oxyd of manganese or oxyd of iridium; bluish black with a mixture of oxyd of cobalt and of manganese, and smoky grey with chloride of platinum. Rose is obtained by diluting, in glaze, gold which has been dissolved in aqua regia. For the blues, oxyd of zinc and alumina may be mixed with the oxyd of cobalt. For greens may be mixed oxyd of cobalt and oxyd of chromium."

"The matters which enter into the composition of the fluxes and which cause the adherence of the metallic oxyds are quartz, feldspar, borax and boracic acid, nitre, the carbonates of potash and of soda, red lead and litharge, and oxyd of bismuth. At Sevres, they employ, for under-glaze painting, seven kinds of fluxes, which suffice for all the colors. The majority of these fluxes are composed of quartz, oxyd of red lead and of boracic acid, and to some is added a small quantity of carbonate of soda.

"The seven fluxes of Sevres are, first, the flint flux, which is prepared by melting quickly in a crucible and then flowing upon a metallic plate a mixture of 3 parts of red lead or of litharge, and 1 part of Etampes sand.

"Second. The flux of greys prepared with 6 parts of red lead or litharge, 2 parts of Etampes sand, and 1 part of pulverized borax.

"Third. The flux of carmines is made of 1 part red lead, 3 parts of Etampes sand, and 5 parts of pulverized borax.

"Fourth. The flux of purple is made with 3 parts of red lead or of litharge, 1 part of Etampes sand, and 5 parts of crystallized boracic acid.

"Fifth. The flux of violets is made of 27 parts of litharge or red lead, 2 parts of Etampes sand, and 11 parts of crystallized boracic acid.

"Sixth. The flux of greens employed, as well as the two preceding M. Salvetat, was prepared by him with 8 1-9 parts of red lead or litharge, 1 part of Etampes sand, and 2 parts of crystallized boracic acid.

"Seventh. Last the flux of the metallic substances, which is of sub-nitrate of bismuth, obtained by decomposing in water the nitrate of the acid of bismuth, to which is added 1-12 of borax."

These fluxes are mixed with the basic oxyds in greater or less proportion, according as the colors are intended for work, which will require a greater or less degree of heat in firing.

Underglaze colors in powder suitable for painting on pottery, can be procured of several different manufactures. As mentioned before, the variety of these colors is not so great as is to be found in overglaze colors, but as they can be readily mixed the number is sufficient for all practical purposes.

In my own work I have made almost exclusive use of French colors, those manufactured by M. Lacroix. These colors are finely ground and of considerable variety of tint. A list of those which have been tried and found to give satisfactory results, may be given here:

There are many others, but these are mentioned as among those that have been tried and found to fire well, and the colors given in this list will be sufficient for the production of all the tints needed in underglaze painting. In faience painted after the Haviland method, fine white clay takes the place of white.

Phillips' English colors are also very good. His mazarine blue, especially, is one of the finest blues I have ever met with. Emery's and Harrison's colors fire very well, but are not so finely ground as the first mentioned.

PAINTING ON POTTERY AFTER THE MANNER OF THE HAVILAND OR LIMOGES FAIENCE.

This method of decorating pottery, although not involving the use of any new principle, was yet so entirely novel an application of principles already in use, as to entitle M. Laurin to all the credit attaching to a very original and important discovery. It places in the hands of the painter of pottery a method at once so artistic, and so thoroughly in accord with the modern school, as to awaken a profound interest in the minds of all lovers of art. It is probable that there are capabilities in this art that have not yet been brought out. It is still in its infancy, and that there are in it possibilities of much importance to the artistic world, can hardly be doubted.

If it were not for the technical difficulties which surround all work on pottery, and this style in particular, it would offer facilities for the production of works of art unequaled by any method heretofore in use. It is the hope of the writer that the following description of the method of decorating pottery, in this manner, may throw some light upon these technical difficulties, and also that artists of ability may be induced to try it, and so demonstrate the capability it undoubtedly possesses.

The mere knowledge of the materials used will, however, no more produce artistic work, than a box of Winsor & Newton's colors, in the hands of a beginner, will enable him to paint a picture, equal to one by Titian. To produce good work in this method, there must be a certain amount of skill at the command of the painter, just as the same degree of skill is requisite in the production of a good picture by any other method. There is a certain boldness of effect produced by the very nature of the materials and process, which probably would not be seen in the work of the same person in other methods, yet the lack of artistic feeling and ability will be as painfully apparent in this as in any other.

Colors may be daubed upon pottery, as they are, alas! upon canvas, by those whose training and whose feeling for art would hardly fit them to become good house painters; but the result will not be good art, nor will it ever be its own excuse for being. If other branches of decorative art require taste, knowledge, and practical skill, so much the more does this, when it offers scope for the highest capacity. To the artist of ability sufficient to make use of it, it furnishes a palette which, although not of the same range as that of oil colors, yet affords an almost unlimited scale of colors, each of which is enhanced to the fullest degree by the brilliant glaze, with which the work is finished. The painting executed with these beautiful colors, moreover, is practically unchangeable, and none of the ravages of time, short of the destruction of the piece of ware itself, can affect it. In decorations for buildings, or for ordinary use in portraiture, or the higher forms of art, it offers, what has long been desired among artists and art lovers, a method of making works of art indestructible and beyond the possibility of change.

The ware known among dealers under the name of "barbotine," has some resemblance to that which we have been considering. It has a light body, which has been subjected to a very slight fire, and is covered with a soft glaze, which ensures great brilliancy of coloring. The ware, however, has no durability, and is a substance that would not stand the action of the elements, if used in external decoration.

METHOD OF PAINTING FAIENCE UNDER THE GLAZE WITH THE USE OF BODY COLOR.

The method of decorating pottery here described is similar in its effects to what is known as the Haviland, or Limoges faience, and is given as the result of numerous experiments made by the writer.

It is somewhat difficult, indeed, almost impossible, to give a correct idea of the palettes to be used in this kind of painting. If colors could be procured, already prepared for use, as oil paints are, and these colors had the same appearance after firing as before, it would be comparatively an easy task. As it is, the colors must be mixed with clays in certain proportions, and, on account of the change produced by firing, the proportions necessary to produce the intensity of color desired, can only be determined from experience. The result, after the work is finished, differs from its appearance before firing to a greater extent than in any other kind of decoration upon pottery. As a rule, it may be said that the colors are intensified by firing. This is also true of other kinds of under-glaze painting, but not in so great a degree as in the case in question. The harmonies and contrasts of color can be kept only in the mind of the artist, and every part of the work must be done with a view to the result when finished, which, as has been said, will differ materially from its appearance during the progress of the painting. Experience only, can give an accurate idea of these changes. This, however, is not an insurmountable difficulty; care and patience added to the requisite artistic ability, will soon lead to satisfactory results.

Before describing the method of painting, it may be well to consider the kind of clay of which the body of the ware should be made, and the state in which the painting should be applied. As to the clay forming the body of the ware, it must be of such a nature as to adapt itself to a glaze sufficiently soft to preserve the colors. As the clay used in the painting must, of necessity, in most cases, be white, in order that the purity and beauty of the colors may not be affected by admixture with it, a body of the same, or very nearly resembling it, would, in some respects, be the best, one reason for this being that the applied clay would be more certain to adhere firmly to the body, both having the same qualities. The proper glaze for such a body would, however, require too great a degree of heat in the firing, and none but the strongest colors could bear it without injury.

In order, therefore, that the glaze may be perfectly adapted to the body of the ware, and yet require no greater degree of heat than the colors used in the painting will bear, it is best to use a body formed from materials which do not need to be fired at a very high temperature. This desired quality is found in some of the natural colored clays. A mixture partaking of the qualities of both yellow and red clay, without the unpleasant color of the former, and possessing greater strength than the latter, has been found to be the best.

THE STATE OF THE WARE IN WHICH THE PAINTING SHOULD BE APPLIED.

For this method of decoration the ware should be in the "green" state. The more moisture it contains the better, and it will be in proper condition if procured as soon as it leaves the hands of the molder, or as soon as it has become sufficiently dry to retain its shape. Pieces can be kept in this state a considerable length of time, by placing them in a box which has been lined with plaster of paris. This lining should be about an inch thick, and if the inside is occasionally sprinkled with water, it will remain moist, and keep the clay in good condition. A box of this kind is really invaluable for all work, where it is necessary to keep clay moist. Within certain limits the clay is improved rather than injured by keeping in this way; the effect upon it being that it becomes tougher and more plastic. If kept too long, it becomes "rotten," and the plastic quality is lost, but it will keep a reasonable length of time, even two or three months. If the box can not be had, the pieces may be covered by a rubber cloth, which should be kept from touching them by a frame of wood or galvanized iron wire, damp cloths having been placed around them to produce sufficient moisture.

It is necessary to exercise this care in keeping the ware moist, in order to prevent the clay, which is applied to the surface in painting, from cracking during the process of drying. It must always be borne in mind that the material made use of in painting is simply clay, which has been artificially colored, and, as clay, becomes subject to the rules which govern all work of this kind, whether it is making pottery, modeling, or painting faience.

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